The present invention relates to the packaging of integrated circuits (ICs) and more particularly to a method of attaching a solder ball to a bonding pad and a ring therefore.
Many types of semiconductor packages use solder balls for package-to-package or package-to-board electrical connections. Many such packages use an array of solder balls, known as a Ball Grid Array (BGA). In order to achieve acceptable reliability, BGA packages must have a predetermined solder ball stand-off height. However, as package density and the number of I/Os increases, the size of the solder ball is constrained to achieve specified pad pitch. For example, increasing ball size may cause solder ball bridging problems during ball attach reflow. That is, solder balls may collapse, creating interconnects between adjacent balls.
FIG. 1 shows a conventional bonding pad 10 including flux 12 and a solder ball 14. The pad 10 typically has a pitch (distance from the center of the pad to the center of an adjacent pad) of about 0.65 mm. A normal size of the solder ball 14 is about 0.33 mm for a 0.65 mm pitch, and after reflow, the height of the ball 14 is reduced to about 0.27 mm, resulting in a low stand-off. However, if a larger ball is used, the aforementioned bridging problems may occur.
It would be advantageous to be able to increase solder ball stand-off height and at the same time, eliminate bridging problems.